1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor for a magnetic read head, more specifically to the use of synthetic exchange biasing to reduce the magnetic track width (MRW) of such a sensor.
2. Description of the Related Art
Magnetic read heads whose sensors make use of the giant magnetoresistive effect (GMR) in the spin-valve configuration (SVMR) base their operation on the fact that magnetic fields produced by data stored in the medium being read cause the direction of the magnetization of one layer in the sensor (the free magnetic layer) to move relative to a fixed magnetization direction of another layer of the sensor (the fixed or pinned magnetic layer). Because the resistance of the sensor element is proportional to the cosine of the (varying) angle between these two magnetizations, a constant current (the sensing current) passing through the sensor produces a varying voltage across the sensor which is interpreted by associated electronic circuitry. The accuracy, linearity and stability required of a GMR sensor places stringent requirements on the magnetization of its fixed and free magnetic layers. The fixed layer, for example, has its magnetization “pinned” in a direction normal to the air bearing surface of the sensor (the transverse direction) by an adjacent magnetic layer called the pinning layer. The free layer is magnetized in a direction along the width of the sensor and parallel to the air bearing surface (the longitudinal direction). Layers of hard magnetic material (permanent magnetic layers) or laminates of antiferromagnetic and soft magnetic materials are typically formed on each side of the sensor and oriented so that their magnetic field extends in the same direction as that of the free layer. These layers, called longitudinal bias layers, maintain the free layer as a single magnetic domain and also assist in linearizing the sensor response by keeping the free layer magnetization direction normal to that of the fixed layer when quiescent. Maintaining the free layer in a single domain state significantly reduces noise (Barkhausen noise) in the signal produced by thermodynamic variations in domain configurations. A magnetically stable spin-valve sensor using either hard magnetic biasing layers or ferromagnetic biasing layers is disclosed by Zhu et al. (U.S. Pat. No. 6,324,037 B1) and by Huai et al. (U.S. Pat. No. 6,222,707 B1).
The importance of longitudinal bias has led to various inventions designed to improve the material composition, structure, positioning and method of forming the magnetic layers that produce it. One form of the present art provides for sensor structures in which the longitudinal bias layers are layers of hard magnetic material (permanent magnets) that abut the etched back ends of the active region of the sensor to produce what is called an abutted junction configuration. This arrangement fixes the domain structure of the free magnetic layer by magnetostatic coupling through direct edge-to-edge contact at the etched junction between the biasing layer and the exposed end of the layer being biased (the free layer). Another form of the present art, patterned exchange bias, appears in two versions: 1) direct exchange and 2) synthetic exchange. Unlike the magnetostatic coupling resulting from direct contact with a hard magnetic material that is used in the abutted junction, in exchange coupling the biasing layer is a layer of ferromagnetic material which overlays the layer being biased, but is separated from it by a thin coupling layer of non-magnetic material. This non-magnetic gap separating the two layers produces exchange coupling between them, a situation in which it is energetically favorable for the biasing layer and the biased layer assume a certain relative direction of magnetization. In direct exchange coupling, the material used to form the gap (eg. Cu or Ru) and its thickness are chosen to allow a ferromagnetic form of exchange coupling wherein the biasing and biased layers have the same directions of magnetization. In synthetic exchange coupling, the non-magnetic material of the coupling layer (eg. Cu, Ru or Rh) and its thickness are chosen to allow antiferromagnetic coupling, wherein the magnetization of the biasing and biased layers are antiparallel. Xiao et al. (U.S. Pat. No. 6,322,640 B1) disclose a method for forming a double, antiferromagnetically biased GMR sensor, using as the biasing material a magnetic material having two crystalline phases, one of which couples antiferromagnetically and the other of which does not.
As the area density of magnetization in magnetic recording media (eg. disks) continues to increase, significant reduction in the width of the active sensing region (trackwidth) of read-sensors becomes necessary. For trackwidths less than 0.2 microns (μm), the traditional abutted junction hard bias structure discussed above becomes unsuitable because the strong magnetostatic coupling at the junction surface actually pins the magnetization of the (very narrow) biased layer (the free layer), making it less responsive to the signal being read and, thereby, significantly reducing the sensor sensitivity. This adverse pinning effect is discussed by Fukuzawa et al. (U.S. Pat. No. 6,118,624) who provide a mechanism for alleviating it by use of a hard magnetic biasing film which has a higher saturation magnetism than the free layer being biased.
Under very narrow trackwidth conditions, the exchange bias method becomes increasingly attractive, since the free layer is not reduced in size by the formation of an abutted junction, but extends continuously across the entire width of the sensor element. FIG. 1 is a schematic depiction of an abutted junction arrangement and FIG. 2 is an equally schematic depiction of a direct exchange coupled configuration. As can be seen, the trackwidth in the abutted junction is made narrow by physically etching away both ends of the sensor, whereas in the exchange coupled sensor, the trackwidth is defined by placement of the conductive leads and bias layers while the sensor element retains its full width.
The direct exchange biasing also has its shortcomings when used in a very narrow trackwidth configuration because of the weakness of the pinning field, provided to the free and biasing layers by the antiferromagnetic layer, which pinning field is found to be, typically, approximately 250 Oe.
A stronger pinning field, typically exceeding 700 Oe, can be obtained using the synthetic exchange biasing method. Such a pinning field can be obtained, for example, using a GMR configuration (numerals referring to angstroms) with a CoFe (10)/NiFe(20) ferromagnetic free bilayer, a Ru (7.5) non-magnetic coupling layer and a CoFe (15) biasing layer antiferromagnetically pinned by an antiferromagnetic IrMn (40) layer. According to micromagnetic simulation, a magnetic trackwidth of 0.15 μm can be obtained with a physical track width of 0.1 μm by using such a configuration. Unfortunately, even with its higher degree of exchange coupling this configuration produces an undesirable level of side reading (sensor response generated by signals originating outside of the magnetic trackwidth region) which is produced by the portion of the free layer that is beneath the biasing layer and conduction leads.
The present invention, therefore, addresses this significant problem of reducing the undesirable side reading of a synthetic exchange biased sensor, while retaining the strong coupling between the biasing layer and the free layer which is responsible for the generally excellent performance of the sensor configuration.